Improved x64 encoder

src/build/arch/x64/encode.go

@@ -0,0 +1,27 @@
+package x64
+
+// encode is the core function that encodes an instruction.
+func encode(code []byte, w byte, mod byte, reg byte, rm byte, opCodes ...byte) []byte {
+	r := byte(0) // Extension to the "reg" field in ModRM.
+	x := byte(0) // Extension to the SIB index field.
+	b := byte(0) // Extension to the "rm" field in ModRM or the SIB base (r8 up to r15 use this).
+
+	if reg > 0b111 {
+		r = 1
+		reg &= 0b111
+	}
+
+	if rm > 0b111 {
+		b = 1
+		rm &= 0b111
+	}
+
+	if w != 0 || r != 0 || x != 0 || b != 0 {
+		code = append(code, REX(w, r, x, b))
+	}
+
+	code = append(code, opCodes...)
+	code = append(code, ModRM(mod, reg, rm))
+
+	return code
+}